1. Field of the Invention
The invention relates to an injection device for receiving a liquid medicament injectable into a human or animal body by means of a needle, which device comprises the following parts: A) a disposable lower part comprising an ampoule holder for accommodating a container containing the liquid medicament, said container having an axially movable plug having a cross-section F for ejecting the liquid medicament from the container, a kinematic device for axially driving the plug forward by a number N of individual steps x, N being a natural number, B) a reusable upper part comprising a casing being detachably connectable to the ampoule holder of the lower part, dosing means for selecting the number N of individual steps x, ejection means for activating the kinematic device of the lower part which effects selectable ejection of a Volume V=NxF of the liquid medicament from the container.
2. Description of the Prior Art
Due to their specific form and analogy to a writing pen, such devices are generally referred to as "injection pens" or just "pens".
According to the prior art, there are essentially two different types of such injection pens.
Examples for devices according to the first group which are intended for one use only are described in detail in the documents EP-B 327 910, EP-A 496 141 and U.S. Pat. No. 5,295,976. In these devices, the ampoule which contains the liquid medicament is incorporated directly in the injection pen and delivered to the patient as a unit. Consequently, these devices are disposed of as a whole when exhausted which, owing to the more complicated parts of the pen which may, for example, also comprise a dosage and metering device, is rather costly. Devices of this group are therefore very expensive nor do they offer any solution for reusing single parts due to the different materials used therein.
Examples for devices of the second group which may be reused are described in detail in the documents DE-C 36 38 984, EP-A 498 737 and U.S. Pat. No. 4,883,472. In these devices, the exhausted ampoule may be replaced by a fresh one. However, the exchange procedure for the ampoule is rather complicated in these known devices. In order to insert a new ampoule into the device, the advance drive mechanism thereof must be returned to a defined starting position (for example by turning the plunger rod equipped with a thread). In order to be able to safely carry out this procedure, the patient must be instructed with due care. Another disadvantage resides in the risk of malfunctions of the dosage mechanism since said mechanism will soon be worn through frequent use.